1. Field of the Invention
The present invention relates generally to implantable medical devices, and more particularly to a cardiac lead incorporating a sleeve composed of a shape-memory polymeric material capable of deforming following implantation into the body.
2. Description of the Related Art
Implantable tubular structures, such as catheters, cardiac stimulator leads, shunts, and others are presently used for a variety of diagnostic and therapeutic purposes. The particular shape indicated for a given implantable tubular structure is dictated in large part by the anticipated physiological structures that will be encountered by the structure. For example, an endocardial lead attached distally to the ventricular apex may have a relatively straight shape. However, other situations may call for a more particularly shaped implantable tubular structure. For example, cardiac stimulator leads intended for implantation in the right atrium or coronary sinus are advantageously provided with one or more bends in the tubular structure to either facilitate entry of the tubular structure into an obliquely disposed passageway, such as the coronary sinus, or to enable the tubular structure to conform with and thereby engage the internal structure of the right atrium to anchor the tubular structure in a particular position.
Presently, some effort is made by medical device manufacturers to supply implantable tubular structures with preselected shapes intended to accommodate particularized physiological structures. These commercially available devices are often supplied in different lengths, and with a variety of different types of preestablished bends. The sizes and preestablished shapes of these conventional devices are typically based on some empirically determined norm for the size and shape of the average anatomical structure to be encountered by the implantable device. The disadvantage associated with using conventional off-the-shelf implantable tubular devices is that internal anatomy varies greatly among individual patients and these systems cannot be deformed in situ. For this reason, a cardiac stimulator lead that may fit one patient may not be suitable for another patient. For those patients whose internal anatomy differs significantly from the hypothetical norm, the use of an off-the-shelf implantable tubular device may represent a compromise that, while not necessarily life threatening, may nevertheless involve a less than optimal treatment regimen.
For example, some patients present serious arrhythmia conditions that stem from highly localized electrical disruptions in the left side of the heart. In some of these cases, epicardial or right ventricular endocardial cardiac stimulation may not provide electrical stimulation that is localized enough to treat the arrhythmia; endocardial stimulation via the coronary sinus may be indicated. Regardless of the ultimate endocardial site requiring stimulation, an endocardial cardiac stimulator lead is normally implanted into the heart by passing a lead through the superior vena cava and into the right atrium. The pathway from the superior vena cava to the right atrium is relatively straight. However, the coronary sinus ostium is located approximately at a 90.degree. angle from that straight pathway. If coronary sinus implantation is indicated, the lead must negotiate the 90.degree. turn to successfully enter the coronary sinus ostium. If the ultimate implantation site for the tip of the lead is within one of the tributaries of the great cardiac vein, the lead must undergo one or possibly several more substantial changes in direction before reaching the targeted tissue site. A conventional lead with permanent bends may prove difficult to navigate to the targeted tissue site under such conditions. There are numerous examples of other types of anatomical structures where such significant changes in direction are associated with the implantation pathway.
One conventional device for imparting a bend in a catheter incorporates a bellows joint for establishing the bent shape. The capability of this system to customize an implantable tubular device is limited in several aspects. First, the types of shapes that may be imparted are normally limited to two dimensions. Second, the portion of the catheter that may be shaped is limited to the immediate vicinity of the bellows joint.
Another conventional lead system employs a mechanical linkage between the lead tip and the proximal end of the lead to enable the lead tip to be selectively curved or straightened by hand manipulating a stylet-like handle connected proximally to the linkage. This system provides a limited ability to manipulate the position of the lead tip in situ. However, the system is complex, costly to manufacture, and because of the space requirements of the linkage, presents a lower limit to the minimum possible outer diameter for the lead.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.